nvptx: Use 'enum ptx_version', 'enum ptx_isa' instead of 'int'

This allows getting rid of the respective type casts.  No change in behavior
intended.

	gcc/
	* config/nvptx/gen-opt.sh: Use 'enum ptx_isa' instead of 'int'.
	* config/nvptx/nvptx-gen.opt: Regenerate.
	* config/nvptx/nvptx.opt: Use 'enum ptx_version' instead of 'int'.
	* config/nvptx/nvptx-opts.h (enum ptx_isa): Add 'PTX_ISA_unset'.
	(enum ptx_version): Add 'PTX_VERSION_unset'.
	* config/nvptx/nvptx-c.cc (nvptx_cpu_cpp_builtins): Adjust.
	* config/nvptx/nvptx.cc (default_ptx_version_option)
	(handle_ptx_version_option, nvptx_option_override)
	(nvptx_file_start): Likewise.